fix code style

Author: PeterPetrik

.github/workflows/code_style.yml

@@ -9,5 +9,5 @@ jobs:
       - uses: actions/checkout@v2
       - uses: psf/black@stable
         with:
-          options: "--check --verbose -l 120"
+          options: "--check --diff --verbose -l 120"
           src: "./mergin"